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From:   Brian Geffon <bgeffon@google.com>
Date:   Thu, 13 Feb 2020 10:20:44 -0800
Message-ID: <CADyq12wWOhGDeUeOB74dxuRKjPhduMWZLBMxOxpm5-yHOpjaRw@mail.gmail.com>
Subject: Re: [PATCH v4] mm: Add MREMAP_DONTUNMAP to mremap().
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Linux API <linux-api@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Will Deacon <will@kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Sonny Rao <sonnyrao@google.com>,
        Minchan Kim <minchan@kernel.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Yu Zhao <yuzhao@google.com>,
        Jesse Barnes <jsbarnes@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Florian Weimer <fweimer@redhat.com>
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Hi Kirill,

> But if you do the operation for the VM_LOCKED vma, you'll have two locked
> VMA's now, right? Where do you account the old locked vma you left behind?

You bring up a good point. In a previous iteration of my patch I had
it clearing the locked flags on the old VMA as technically the locked
pages had migrated. I talked myself out of that but the more I think
about it we should probably do that. Something along the lines of:

+    if (vm_flags & VM_LOCKED) {
+      /* Locked pages would have migrated to the new VMA */
+      vma->vm_flags &= VM_LOCKED_CLEAR_MASK;
+      if (new_len > old_len)
+              mm->locked_vm += (new_len - old_len) >> PAGE_SHIFT;
+   }

I feel that this is correct. The only other possible option would be
to clear only the VM_LOCKED flag on the old vma leaving VM_LOCKONFAULT
to handle the MCL_ONFAULT mlocked situation, thoughts? Regardless I'll
have to mail a new patch because that part where I'm incrementing the
mm->locked_vm lost the check on VM_LOCKED during patch versions.

Thanks again for taking the time to review.

Brian